Discharge Trading Programs Based on Constructed Wetlands for Nutrient Trading

High nutrient concentrations in surface waters, specifically those of nitrate and phosphorus, are a major cause of hypoxia. It is likely that in the near future, point source dischargers will be required to limit their nutrient discharges. However, the cost of doing so is not trivial. A putatively cheaper alternative is to remove the nutrients in constructed wetlands. The removal credits generated can then be sold to dischargers who require them to meet regulatory standards. In the present work, the tradeoff between cost and wetland removal capacity was estimated, so to assess the feasibility of such a nutrient trading program. The analysis was carried out for two potential sites in east-central Illinois. One of the wetlands is adjacent to a municipal wastewater treatment plant and receives relatively concentrated effluent flow from the plant. The other is downstream from the plant and receives stream water. The costs of operating the wetlands individually, and concurrently, were estimated. It was found that for the most part, phosphorus removal is the binding constraint that determines the design of a wetland system, and that a two-wetland design is more cost-effective than a single-wetland design. Further, nutrient trading could potentially save up to 75% over conventional in-plant treatment.